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From Knut Anders Hatlen <knut.hat...@oracle.com>
Subject Re: How Much Memory for hash joins
Date Mon, 04 Mar 2013 13:21:42 GMT
Mike Matrigali <mikem_app@sbcglobal.net> writes:

> Also note that these are all estimates within the system.  As Knut
> pointed out there are some known problems with the estimates.  And
> even with fixes he has suggested, the code is probably just guessing
> with things like blobs/clobs.

I haven't verified, but I think HashJoinStrategy uses
DataTypeDescriptor.estimatedMemoryUsage() to estimate how much memory
the hash table will consume. That method has no case for BLOB or CLOB,
so it looks as if it will return zero for LOB columns. If that's so, it
will definitely overestimate how many rows fits in maxMemoryPerTable
kilobytes if the rows contain LOBs.

Knut Anders

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